The vast majority of chemical processes to generate chlorine dioxide currently in commercial use in pulp mills take place under sub-atmospheric conditions. In such processes, chlorine dioxide is generated by reducing sodium chlorate using a reducing agent, such as methanol or hydrogen peroxide, in an aqueous acid reaction medium containing sulfuric acid maintained at its boiling point under a sub-atmospheric pressure. The reaction medium in such processes is a slurry with a solids content commonly ranging from about 10 to about 30% w/w. The two major parameters in terms of the concentration control in the reactor, namely chlorate molarity and titratable acidity, are normally maintained within target bands no wider than +/−about 0.2 M and +/−about 0.2 N, respectively. Such control targets are accomplished based on analytical results of chemical tests performed manually by plant operators.
Being able to continuously monitor liquor composition is desirable beyond the obvious manpower savings. The accuracy and precision of process control that would be achieved by means of on-line measurement would ultimately lead to higher reaction efficiency and lower chemical consumptions. But all known attempts to meet the above analytical requirements without human intervention have not been successful or not in sustainable and reliable services. One of the reasons is the difficulty inherent to the sampling of hot and saturated slurry-type process streams.